The present application relates to a display apparatus and a display method, and more particularly to an apparatus for and a method of displaying an image by projecting light onto a screen.
Technologies for projecting image light onto a screen to display an image on the screen have found widespread use in recent years.
Among those technologies are a variety of display systems for generating a two-dimensional image by scanning a one-dimensional image which has been generated by a modulated laser beam and projecting the two-dimensional image onto a screen to display the image on the screen.
One conventional display apparatus of the type referred to above will be described below with reference to FIGS. 1 to 5 of the accompanying drawings.
FIG. 1 is a block diagram of a conventional display apparatus 1. The display apparatus 1 has a light source 11, a light modulator 12, a projecting optical system 13, a light deflector 14, and a magnifying projection system 15. The display apparatus 1 projects image light onto a screen 16 to display an image on the screen 16.
The light source 11 emits a laser beam and applies the laser beam to the light modulator 12.
The light modulator 12 modulates the laser beam emitted from the light source 11. Specifically, the light modulator 12 modulates the laser beam emitted from the light source 11 with an image signal that is supplied to the light modulator 12 to form bright and dark image areas.
The projecting optical system 13 reflects the light beam applied from the light modulator 12 to form a projection image.
The light deflector 14 scans the light beam applied from the projecting optical system 13 to form a second image.
The second image produced by the light deflector 14 is magnified and projected by the magnifying projection system 15 onto the screen 16. Specifically, the magnifying projection system 15 magnifies and projects the second image, which is produced as an intermediate image by the light modulator 12, the projecting optical system 13, and the light deflector 14, onto the screen 16.
The display apparatus 1 displays an image on the screen 16 in the manner described above.
FIG. 2 schematically shows the light source 11, the light modulator 12, the projecting optical system 13, the light deflector 14, and a magnifying projection system entrance unit 25 which is an entrance unit of the magnifying projection system 15. FIG. 3 schematically shows light paths in the light modulator 12, the projecting optical system 13, the light deflector 14, and the magnifying projection system entrance unit 25.
As shown in FIG. 2, a condensing optical system 21 is provided for condensing the light beam from the light source 11 and applying the condensed light beam to the light modulator 12. Though the condensing optical system 21 is shown as including a single lens in FIG. 2, it actually is arranged to apply a linear beam, which extends along an array of light modulating elements of the light modulator 12, to the light modulator 12.
The projecting optical system 13 includes a main mirror 22 and an auxiliary mirror 23 disposed in confronting relation to the main mirror 22. The main mirror 22 reflects the light beam from the light modulator 12 to the auxiliary mirror 23, and reflects again a light beam reflected from the auxiliary mirror 23 to a scanning mirror 24 of the light deflector 14. The auxiliary mirror 23 reflects the light beam from the main mirror 22 back to the main mirror 22.
The scanning mirror 24 of the light deflector 14 reflects the light beam from the projecting optical system 13 while it is being angularly moved through a predetermined angle about a given axis. The scanning mirror 24 scans an image that is formed by the light beam modulated by the light modulator 12 and reflected by the projecting optical system 13, forming a two-dimensional image 26 in the magnifying projection system entrance unit 25 which is an entrance unit of the magnifying projection system 15.
FIG. 4 shows in block form the light deflector 14 of the conventional display apparatus 1.
As shown in FIG. 4, the light deflector 14 includes the scanning mirror 24, a galvanometer motor 31, a rotary encoder 32, a motor control circuit 33, and a motor driver circuit 34.
The galvanometer motor 31 is energized by the motor driver circuit 34 to angularly move the scanning mirror 24 that is coupled to the galvanometer motor 31 by a shaft thereof. The rotary encoder 32 is coupled to the galvanometer motor 31 by another shaft thereof which is coaxial with the first-mentioned shaft. The rotary encoder 32 detects an angle of the first-mentioned shaft, i.e., the scanning mirror 24, and supplies a signal representing the detected angle to the motor control circuit 33. The term “rotary encoder” generally refers to a sensor for detecting an angle of a shaft. The motor control circuit 33 supplies a control signal for controlling the energization of the galvanometer motor 31 to the motor driver circuit 34 based on the signal supplied from the rotary encoder 32 as representing the angle of the first-mentioned shaft, i.e., the scanning mirror 24. Specifically, the motor control circuit 33 compares a preset shaft angle and the shaft angle represented by the signal supplied from the rotary encoder 32, and supplies a control signal depending on the difference between the compared angles, i.e., an error, to the motor driver circuit 34.
Based on the control signal supplied from the motor control circuit 33, the motor driver circuit 34 supplies a current for energizing the galvanometer motor 31 to the galvanometer motor 31, thereby energizing the galvanometer motor 31.
As described above, the light deflector 14 of the conventional display apparatus 1 detects an angle of the scanning mirror 24 and angularly moves the scanning mirror 24 based on the detected angle of the scanning mirror 24.
The scanning mirror 24 may include a so-called polygon mirror (polygonal mirror). If the scanning mirror 24 includes a polygon mirror, then the galvanometer motor 31 is replaced with a constant-speed motor for rotating the polygon mirror. The constant-speed motor has a shaft rotatable at a constant rate in a constant direction to rotate the polygon mirror. The rotary encoder 32 is coaxial with the shaft of the constant-speed motor and detects an angle of the shaft of the constant-speed motor.
If the polygon mirror and the constant-speed motor are employed, then the motor control circuit 33 supplies a control signal for controlling the energization of the constant-speed motor to the motor driver circuit 34 based on the signal supplied from the rotary encoder 32 as representing the angle of the shaft of the constant-speed motor. The motor driver circuit 34 energizes the constant-speed motor based on the control signal supplied from the motor control circuit 33.
However, various problems arise when the rotary encoder 32 detects an angle of the scanning mirror 24.
FIG. 5 shows various factors for producing errors between an image projected by the conventional display apparatus 1 onto the screen 16 and the angle of the shaft of the galvanometer motor 31
Since the scanning mirror 24 is mounted on the shaft of the galvanometer motor 31, a difference (error) is developed between the angle of the shaft of the galvanometer motor 31 and the angle of the scanning mirror 24 due to a misalignment between the shaft of the galvanometer motor 31 and the scanning mirror 24. A difference (error) is also developed between the angle of the shaft of the galvanometer motor 31 and the angle of the scanning mirror 24 because of a torsion and resonance of the shaft of the galvanometer motor 31 and the scanning mirror 24.
Specifically, the scanning mirror 24 scans an image projected from the projecting optical system 13 at an angle which is different (due to an error) from the angle of the shaft of the galvanometer motor 31, thereby forming a two-dimensional image, and applies the formed two-dimensional image to the magnifying projection system entrance unit 25.
Stated otherwise, an image that is affected by an error which is developed between the angle of the shaft of the galvanometer motor 31 and the angle of the scanning mirror 24 due to a misalignment, torsion, and resonance of the shaft of the galvanometer motor 31 and the scanning mirror 24, is projected by the magnifying projection system entrance unit 25 onto the screen 16.
A similar problem exists with respect to the control loop for controlling the galvanometer motor 31, i.e., the rotary encoder 32, the motor control circuit 33, and the motor driver circuit 34. Specifically, since the rotary encoder 32 is coaxial with the shaft of the galvanometer motor 31 for angularly moving the scanning mirror 24, a difference (error) is developed between the angle of the shaft of the galvanometer motor 31 and the angle of the shaft of the rotary encoder 32 due to a misalignment between the shaft of the galvanometer motor 31 and the shaft of the rotary encoder 32. A difference (error) is also developed between the angle of the shaft of the galvanometer motor 31 and the angle of the shaft of the rotary encoder 32 because of a torsion and resonance of the shaft of the galvanometer motor 31 and the shaft of the rotary encoder 32.
The angle detected by the rotary encoder 32 contains an error of the rotary encoder 32 and a quantization error produced thereby.
In addition, when the shaft of the rotary encoder 32 is turned through a certain angle, i.e., to a predetermined angular position, a certain period of time needs to be consumed before the rotary encoder 32 outputs a signal representing the certain angle. Therefore, when the rotary encoder 32 detects an angle, a certain time delay exists before the rotary encoder 32 actually outputs a signal representing the detected angle.
As described above, the signal output from the rotary encoder 32 contains errors with respect to the angle of the shaft of the galvanometer motor 31 due to the misalignment, torsion, and resonance of the shaft of the galvanometer motor 31 and the shaft of the rotary encoder 32, the error of the rotary encoder 32 and the quantization error produced thereby, and the error due to the timed delay of the output signal from the rotary encoder 32.
A servo mechanism including the motor control circuit 33, the motor driver circuit 34, and the galvanometer motor 31 controls the angle of the shaft of the galvanometer motor 31 based on the signal, including the above errors, from the rotary encoder 32.
The errors caused by the misalignment, torsion, and resonance of the shaft of the galvanometer motor 31 and the shaft of the scanning mirror 24 are different from the errors caused by the misalignment, torsion, and resonance of the shaft of the galvanometer motor 31 and the shaft of the rotary encoder 32 and the quantization error produced thereby, and the error due to the timed delay of the output signal from the rotary encoder 32. As viewed from the image projected onto the screen 16, the signal output from the rotary encoder 32 is affected by all these errors.
The rotary encoder 32 is highly expensive if it is of high resolution and high accuracy.
If the shaft of the galvanometer motor 31 is extended and the rotary encoder 32 is mounted on the extended shaft, then the moment of inertia of the shaft which turns in unison with the scanning mirror 24 increases. The galvanometer motor 31 reciprocally angularly moves its shaft periodically in alternate directions. If the moment of inertia of the shaft increases, then a large amount of energy is required to accelerate and decelerate the shaft in its turning motion. As a result, it is highly difficult to control the galvanometer motor 31 with the extended shaft.